14 THE LAND SNAILS 
spread a membrane, very appropriately called the tongue, 
or lingual membrane, as the snail uses it in lapping its 
food. This membrane is quite long and broad, and is 
. covered with minute silicious denticles, or teeth, as they 
are called. 
As an object for the microscope, it will repay one the 
trouble attendant on dissecting this membrane from the 
mouth of a snail. A magnified figure of the entire tongue 
is given on plate 1, fig. 6. Nothing can exceed the 
beauty and regularity in the form and arrangement of the 
denticles. These are pointed and turn backwards, thus 
forming a series of little claws and hooks, and are admira- 
bly adapted to perform the rasping function allotted to | 
them ; fig. 1, plate 2, gives a side view of afew of these 
teeth to show their hooked character. The number of den- 
ticles on the tongue is very great. Some species, the 
white-lipped Helix, for instance, having nearly twelve 
thousand denticles. It is difficult to conceive the minute- 
ness of these particles, when we consider that the mem- 
brane on which they rest is not a quarter of an inch long, 
and only half as wide. The denticles are arranged in reg- 
ular longitudinal and transverse rows. Figure 3, plate 1, 
represents two transverse rows of these denticles, and fig. 
4 a central tooth, with lateral teeth more highly magnified 
to show their form. It-will be noticed that the central 
denticles are symmetrical in form, having the two sides 
alike, while those on each side are not symmetrical. 
illustrating the dentition of a species, it is only necessary 
to draw one half of one transverse row, including the cen- 
tral denticle, at the same time mentioning the number of 
transverse rows on the membrane; thus in the white-lip- 
ped Helix, a specimen of which we examined, we found 
eighty-nine denticles in a transverse row, that is, one 
